Emission reduction apparatus

ABSTRACT

An emission reduction apparatus includes first and second exhaust paths carrying respective first and second Nox traps. One trap has a smaller capacity than the other and is operative while the larger trap regenerates, allowing symmetric operation.

The invention relates to an emissions reduction apparatus for examplefor reducing noxious emissions from a vehicle engine exhaust.

One known apparatus of this type comprises a NOx trap for example of thetype manufactured by Johnson Mathey of the United Kingdom. The trapcomprises an exhaust inlet, an outlet and a filter between them having,on an outlet side, a NOx wash coat. The filter can be, for example, aporous ceramic such as ceramic cordierire and the wash coat a NOxabsorbent, for example a precious metal and an alkaline metal dispersedinto an alluminous port. NOx traps of this type in fact carry out a dualfunction; the porous ceramic acts as a particulate filter forparticulate matter in the exhaust steam and the wash coat traps NOx aswell as HC and CO.

In use the NOx trap needs to regenerate frequently, for example every 60to 90 seconds at normal operating temperatures although this will bemuch shorter at low or very high temperatures. Regeneration is typicallyachieved by injecting fuel into the exhaust stream which induces anexothermic reaction in the filter whereby the trapped particulatematerial oxidises. The remaining fuel reacts with the absorbed NOxregenerating the wash coat. The regeneration period is typically 2 to 4seconds. A trap of this type is particularly suited to diesel engineexhausts as it is suited to the lean air-fuel ratio of the exhaust gas,and is often referred to as a lean NOx trap (LNT).

European patent application number EP1055806 discusses an enhancedsystem including two NOx traps in parallel exhausts paths. In operation,at start up a first path is opened and a second path closed and theexhaust is hence diverted through the corresponding first NOx trap. Thesystem identifies when the absorption capacity of the first trap isapproached and enters a regeneration routine. According to this routinea second valve opens allowing the exhaust to pass into the second path,and the first valve is partially closed such that the majority of theexhaust flow is through the second path and NOx path. In practice a flowratio of around 95% to 5% is observed. The remaining flow in the firstpath is sufficient to carry injected fuel to induce regeneration in thefirst trap. When regeneration is detected the first valve is fullyclosed and fuel injection is ceased, and the first trap then remainsidle until the second trap approaches its absorption capacity. Thisprocess is repeated providing the advantage exhaust purification iscontinually carried out, switching from one trap to the other whenregeneration is required. In addition the amount of fuel injected in theregenerating leg is reduced because the exhaust flow is low and theamount of oxygen in that leg therefore low. Also it is easy to achieve ahigh temperature from a fuel induced exothermic reaction in theregenerating leg during regeneration because the mass airflow is low.Low space velocity also stops the completeness of regeneration byensuring sufficient residence time for full NOx conversion to N₂.

A problem that arises with a parallel path system such as this is that,in practice asymmetric operation is observed such that one of the twoarms is favoured and tends to spend proportionally more time in theemissions reduction mode.

The invention is set out in the appended claims. The invention solvesvarious problems with the known arrangement. In particular the observedasymmetry of operation is exploited by having one emissions trapsignificantly smaller than the other such that it is effectively onlycarries out emissions reduction whilst the larger trap is regenerating.This gives rise to significantly reduced costs as the smaller capacityleg has a lower total mass loading and reduced size. Similarly it allowsreduced packaging volume. Yet further it has been observed in the knownsystems that the regenerating leg after completion of the regenerationcools rapidly before it is switched over to trapping so that temperaturemanagement can be a significant concern. In the present case the largertrap only has a short period idle, if at all, after regeneration suchthat minimal temperature drop is observed and hence improved efficiencyis obtained. It will be appreciated that the emissions reductioncapacity can be reduced by vaying any one or more of a number ofparameters including physical size, length, diameter or volume, physicalor thermal mass, chemical formulation or emissions reducing material.

Yet further there is a reduced fuel consumption penalty. In knownsymmetric parallel systems over-injection of fuel may be required in theregenerating leg to keep the temperature up which can overall increasefuel consumption. This problem is clearly reduced in the presentinvention as a result of the provision of the smaller trap and thegenerally improved temperature management. In fact the reduced size ofthe smaller trap requires reduced added fuel during regeneration asthere is less residual oxygen in the leg to remove.

In the preferred embodiment according to which the smaller trap has alower temperature formulation, this matches the temperature regime inthe system according to which the smaller trap will tend to remain at alower temperature allowing better conversion efficiency.

Embodiments of the invention will now be described, by way of example,with reference to the drawings of which:

FIG. 1 is a schematic view of an emission reduction apparatus accordingto the present invention; and

FIG. 2 is a schematic view of a NOx trap including a particulate filter.

Referring to FIG. 1, an engine exhaust path 10 is split into twoparallel paths 12, 14. Each path includes a respective valve 16, 18,operation of which is controlled by a respective control line 20, 22.Downstream of each valve 16, 18 a fuel injection port 22, 24respectively is provided downstream of which are respective NOx traps26, 28. Paths 12 and 14 continue after the respective NOx traps to anyappropriate exhaust outlet 30, 32.

The general configuration of a NOx trap including a particulate filterwill be well known to the skilled reader and so only a very briefdescription is provided here with reference to FIG. 2. In particular thetrap designated generally 40 includes a plurality of passages 42,44, 46,48, 50. The trap has an inlet end designated generally A and an outletend designated B and alternate passages have their inlet end closed andtheir outlet end open and vice versa. For example in the embodimentshown passages 42,46, 50 have their ends closed at the inlet end A andtheir ends open at the inlet end B whilst passages 44 and 46 have theirends open at inlet end A and their ends closed at inlet end B. As aresult exhaust gas entering passages 44 and 48 at inlet end A is forcedto pass through the walls 52 between the passages in order to exit frompassages 42, 46, 50. As discussed above the walls 52 are formed of aporous material to trap particulate matter and coated with a NOx washcoat to absorb NOx.

It will be seen that, according to the invention, the NOx traps 26, 28provided on respective paths 12, 14 are of different capacities in aratio in the region of 5:1 to 10:1. In addition, as discussed in moredetail below in a preferred embodiment the smaller of the traps 28 mayalso have a lower temperature formulation, i.e. be designed to operatemost efficiently at a lower temperature than the larger trap 26.

In operation the path 14 is initially closed by valve 18 and valve 16 isopen diverting flow through path 12 and NOx trap 26. When the trap 26 isfully loaded or approaching fully loaded the regeneration phase isentered according to which valve 18 is opened and valve 16 partiallyclosed so as to divert roughly 95% of the exhaust flow through path 14.Fuel is injected at port 22 and regeneration takes place at the trap 26.Meanwhile the trap 28 operates normally. Once the trap 28 is fullyregenerated fuel injection is ceased and valve 16 fully closed such thatall of the exhaust stream is diverted through the smaller trap 28. Whenfull loading of the smaller trap 28 is sensed the operation is reversed.

The sensing and controlling steps during operation of the cycle arewell-known to the skilled reader and are not discussed in detail here.For example appropriate loading and regenerating sensors can be providedon the traps 26, 28 or appropriate control logic can be implemented inan engine control unit monitoring, for example engine load or engineoutput, or trap temperature to assess when a full load or fullregeneration are achieved. Valves 16 and 18 are then controlled by thecontrol system such as the ECU via a control line 20 and 22, to ensuretheir operation at the correct times.

It will be seen that the asymmetrical capacity of the two traps givesrise to a corresponding asymmetrical operating cycle in which the largertrap tends to remain operative for a significant longer time than thesmaller trap. In fact the smaller trap need only remain operative forthe length of time it takes the larger trap to regenerate although itsoperative period should be designed to be longer than the longestpossible regeneration period of the larger trap to avoid mis-operation.There will also be a temperature asymmetry as the temperature of thelarger trap will build up during its emissions reduction period and alsoduring the correspondingly longer regeneration period. As a result thesmaller trap benefits from a lower temperature formulation so that itsstorage capacity is enhanced at lower temperature.

As a result the invention exploits the observed asymmetry of operationin known physically symmetrical systems. It is believed that theasymmetry in the known systems arises for the following reason: as themajority of the exhaust flow goes through one arm whilst the other armregenerates, the trapping arm remains hot whilst the regenerating armcools rapidly following the short regeneration. When the valves switchthe exhaust gas flows through a comparatively cold trap. Bearing in mindthat the NOx storage capacity of a trap is very temperature dependent,diverting a NOx rich exhaust stream onto a cool trap results in rapidsaturation of the trap (the NOx storage capacity of a trap shows a peaksomewhere in the 300 to 450° C. range depending on formulation); at lowtemperature (and very high temperatures) the storage capacity drops offsharply.

Because the cold leg saturates so rapidly the system switches back tothe other leg before its temperature can come up to its optimum forstorage. As the switch back time has been so short, by the same tokenthe other leg is still hot and so still has high trapping efficiency.Meanwhile the cooler leg undergoes regeneration but because the amountof trap NOx is low the regeneration is short and less fuel is injectedto do this, hence less of an exothermic reaction and less heating.Conversely when the other leg regenerates, as a higher proportion of NOxhas been stored, a higher regeneration temperature is reached. The neteffect is that very quickly a temperature asymmetry arises in operationwhich effectively feeds back, lengthening the operation time of one legand reducing that of the other, even though both traps are identical.

It will be appreciated that the system of the present invention can beadapted in various ways without departing from the inventive concept.For example it can be applied to any type of emission reductionapparatus which has a temperature dependent emission reduction orregeneration regime.

Although discussion is directed in the specific embodiment describedabove towards a NOx trap including a particulate filter it will beappreciated that the invention will work equally well with a NOx trapexcluding such a filter and including a NOx reduction component only.

The respective capacities of the two traps can be varied in terms of thevolume or the temperature or other reduction formulation of the traps.Although a control scheme is described above according to which pathsare switched only when the corresponding trap requires regeneration,instead the path can be switched back from the smaller trap to thelarger trap path as soon as the larger trap has regenerated, that is,before the smaller trap has fully loaded, keeping the larger trap at anoptimum operating temperature.

1. An emission reduction apparatus for an engine exhaust, the apparatuscomprising first and second exhaust paths and first and secondregenerable emission reduction elements in the respective paths in whichthe first emission reduction element has a greater emission reductioncapacity than the second emission reduction element.
 2. An apparatus asclaimed in claim 1 in which the first and second emission reductionelements have at least one of a heat dependent regeneration regime and aheat dependent emission reduction regime.
 3. An apparatus as claimed inclaim 1 in which the emission reduction element comprises a NOx trap. 4.An apparatus as claimed in claim 3 in which the NOx trap includes aparticulate filter.
 5. An apparatus as claimed in claim 1 in which thesecond emission reduction element has a lower operative temperatureformulation than the first emission reduction element.
 6. An emissionreduction system including an apparatus as claimed in claim 1 and acontroller for controlling operation of the apparatus.
 7. An engineincluding an exhaust providing an exhaust path and the system as claimedin claim 6 provided in the exhaust path.
 8. An engine as claimed inclaim 7 comprising a diesel engine.
 9. A vehicle including an engine asclaimed in claim
 7. 10. A method of reducing engine exhaust emissionscomprising switching an engine exhaust stream between first and secondengine exhaust paths having first and second regenerable emissionreduction elements therein in which the exhaust stream is switched to asecond path during regeneration of the regenerable element in the firstpath and then switched back to the first path when regeneration iscomplete.
 11. An apparatus system engine vehicle or method substantiallyas herein described with reference to the drawings.